1. Field of the Invention
The invention relates to a method for continuously controlling the shrinkage of an amorphous film during taking-off from a calender, the shrinkage being measured over the film width and film length with the aid of the dichroism or the birefringence of the film, and an arrangement herefor.
2. Description of the Related Art
One of the conventional methods for processing plastics into films is calendering. In calendering, the material is traverses a nip only once and is brought in the process into a special geometric shape, whereas in rolling the material traverses a nip several times. For this purpose, a calenderable composition is firstly compounded and prepared with additives by mixing. The finished mixtures are plasticized, broken down, homogenized and applied to the calender in the form of a kneading mass. By contrast with the extrusion or injection molding of thermoplastics, in this case the process steps of gelling, homogenizing and shaping take place not inside one machine but one after another in separate units. Rolling out the kneading mass to form a sheet gives the material a preliminary shape with reference to thickness and width. The film is then removed or taken off from the calender, the film being guided over take-off rolls, after which the hot film is cooled, running over chill rolls, and finally relatively long film webs are wound up, or short film pieces are cut into lengths and stacked. During cooling, the film traverses the thermoplastic region, in which tensile stresses lead to orientations and frozen-in stresses inside the film. Films which have been wound up or cut into lengths are further processed by the customer using thermal methods and must have specific shrinkage properties depending on the thermoforming plant used for this purpose and on the thermoforming method applied. Shrinkage therefore constitutes a quality property of amorphous films, it being necessary for the maintenance of the shrinkage to be detected by measurement and to be adhered to by changing process parameters when taking off the film from the calender.
Film shrinkage is based on a molecular orientation which is to be ascribed in the case of calendering to the deformation of the melt in the course of the process and to the shearing of the thermoplastic. Deformations will be produced, inter alia, as follows:
by the plasticizing of the raw material in the kneader, PA1 from the technology of wide flow and from the shearing and elongation of the melt in the nips, PA1 by stretching of the film upon deflection around the take-off rolls, PA1 by stretching of the film in the region of the air gaps between the chill rolls. PA1 fluctuations in the dimensions of the samples owing to the punch which is used to produce the square test piece, PA1 twisting of the punch with respect to the longitudinal direction of the film, PA1 temperature fluctuations in the heating oven, PA1 fluctuations in the conditioning time in the heating oven, PA1 inaccuracies in the determination of the geometric dimensions. PA1 the control of the temperature and speed of the film web when being taken off downstream of the calender, PA1 additives which influence the glass transition temperature of the film, PA1 additives which influence the stretching behavior of the film, and PA1 design measures aimed at reducing relaxation. PA1 .phi./2.PI.=phase difference between the component beams in which the incident light beam is split. PA1 d=thickness of sample. PA1 n.sub.parallel =index of refraction of the component beam parallel to the propagation direction of the incident light beam. PA1 n.sub.perpendicular =index of refraction of the component beam perpendicular to the propagation direction of the incident light beam.
The intensity of the orientation depends on the degree of the deformation, on the temperature profile, on the cooling time and/or the cooling profile, and on various material parameters. As a reaction to the introduction of orientations during production, the films shrink when they have to be heated, for example, upon thermoforming or some other later processing. Thus, for example, rigid PVC films, which generally do not contain plasticizers, but only additives as processing aids, are processed to form thermoformed beakers, containers, dishes, blister packs, sorting bases for fruit, chocolates, confectionery or small industrial parts. Further applications of rigid PVC films are to be found in the building of apparatus and in the automobile industry, in the building sector and in special fields such as, for example, the production of identity cards, such as credit cards, pass cards, key cards, phone cards etc.
Generally speaking, shrinkage denotes the dimensional change, in particular the change in length and width of a film as a consequence of the effect of heat. In the case of conventional shrinkage measurement, the films are stored at the prescribed temperature in a circulating air drying cabinet or in a liquid bath; the shrinkage occurring after a specific time is measured at room temperature. In the DIN Standard 53377, the determination of the shrinkage of a film is laid down in such a way that a square test piece with an edge length of 100 mm is conditioned in an oven for 10 minutes at 140.degree. C. The sample is subsequently measured after having cooled. The difference between the initial dimension before the heat treatment and the final dimension after the heat treatment is determined by length measurement, and the measured value is specified as shrinkage S in %. It then holds for this shrinkage S that ##EQU1##
Other storage times and temperatures in the measurement of the film shrinkage are conditioning over 15 minutes at a temperature of 100.degree. C. and heat treatment over 5 minutes in water at a temperature of between 70 and 95.degree. C. A number of errors can occur in this conventional shrinkage measurement according to DIN, such as:
The said errors lead to a fluctuation of up to 36% in the shrinkage value determined. This substantial inaccuracy brings conventional shrinkage measurement into question, in particular even slight deviations from the stipulated temperature in the heating oven in the interval from 120.degree. C. to 160.degree. C. lead to significant shrinkage deviations. A further disadvantage of conventional shrinkage measurement consists in that it is impossible to take account of the mutual influence of longitudinal, transverse and thickness shrinkage. The traditional description of shrinkage proceeds from a one-dimensional consideration of the change in length. Since, however, films are three-dimensional structures, the change in a dimension must depend on the two other dimensions if the latter are not negligibly small. In the case of a film thickness in the range from 60 .mu.m to 1000 .mu.m, it is certainly permissible to neglect the thickness shrinkage in contrast with the longitudinal and transverse shrinkage. In order to be able to make an unambiguous statement with respect to longitudinal and transverse shrinkage, it is appropriate for the test piece to be punched out in the machine direction of the film in order to measure the longitudinal shrinkage, and for it to be punched out transverse to the machine direction in the case of measuring the transverse shrinkage. In the first case, the width of the test piece is to be small by contrast with the length, while in the second case the length is, conversely, to be small by contrast with the width.
As already indicated above, the cause of the shrinkage resides in the elongation of the film above the glass transition temperature. The elongation orients the molecule chains, which are then frozen by the subsequent cooling to below the glass transition temperature. Upon later reheating above the glass transition temperature, the frozen-in stresses are released again and the molecule chains reassume their original configuration. Restoring forces (shrinkage forces) which cancel the initial deformation are produced as a consequence of the entropy change occurring in such cases. It is known that the orientation of the molecule chains in an amorphous film causes changes in the birefringence and the dichroism of the films, with the result that it is possible by measuring the dichroism or the birefringence to determine the orientation of the molecule chains, thus also to determine the shrinkage properties.
The shrinkage is influenced by
According to the invention, the shrinkage is measured by making use of optical properties of a polymer which are based on anisotropy effects, specifically dichroism and birefringence. Dichroism, also denoted as directionally dependent absorption of polarized light, embodies the second moment of the orientation distribution of molecule chains in a polymer. In this case, the intensity of the absorbed light depends on the vector direction of the incident beam relative to the orientation direction, the polarized light being absorbed to different degrees perpendicular and parallel to the stretching direction.
Birefringence describes the difference between the refractive indices of two mutually perpendicular principal axes, and likewise the second moment of the orientation distribution. Because of the optical anisotropy, that is to say different refractive indices, the light beam incident on a sample is split into two polarized, mutually perpendicular component beams with varying rates of propagation. After the sample has been left, there is a phase difference between the component beams which, when multiplied by the wavelength of the incident light yields the path difference ##EQU2## wherein .GAMMA.=path difference. .lambda.=wavelength of the incident light beam.
The birefringence itself is yielded from EQU .GAMMA./d=n.sub.parallel -n.sub.perpendicular
The orientation of the molecules corresponds to the shrinkage capacity of the polymer or of the film. The transmission behavior of light transilluminating the film changes with the degree of orientation. In the case of rotation of a film which is located between a polarizer and an analyzer, the transmission of the light depends on the twisting angle of the film. In this case, the directions of polarization of the polarization filters, that is to say of the polarizer and analyzer, are perpendicular to one another. In order to improve the intensity of the light transilluminating the film in a specific mutual positioning of the polarizer and analyzer, the orientation of molecule chains in the film can be determined as follows. The film is held in the machine direction parallel to the passing direction of the polarizer or analyzer and subsequently rotated until a maximum value arises at the detector. The angle of rotation of the film is then equal to .eta., and the angle .epsilon. which the molecule chains form with the machine direction is then yielded as EQU .epsilon.=45.degree.-.eta.
If the molecules are orientated in the machine direction, .epsilon.=0, since then the first maximum of the change in intensity is .eta.=45.degree..
DE-C-24 30 521 describes a method and a device by means of which the state of polarization of a beam, that is to say its ellipticity and direction of polarization, can be determined by means of a rotating analyzer, the intensity of the beam passed being plotted as a function of the angle of rotation of the analyzer. Fourier analysis permits the ellipticity parameters to be determined. Also known are arrangements without moving elements which operate with a plurality of fixed detectors. In this case, the main beam to be analyzed is split into a plurality of component beams, or a plurality of detectors are arranged inside a main beam. In DE-A-32 12 809, a grating is used to split the main beam of a laser pulse into two component beams by diffraction into higher orders. This permits the simultaneous measurement of two mutually perpendicular directions of polarization, but this measurement does not suffice to provide a general characterization of the state of polarization of the laser pulse. DE-A-19 51 920 describes an arrangement having four fixed detectors and a cone which is used for beam splitting. However, the adjustment of the four independent beams is very complicated and susceptible to interference.
The position is similar for the analyzer device described in DE-A-39 31 540, in which splitting into six component beams is performed. Four component beams are used to characterize the ellipticity. The beam-splitting polarimeter used here is limited to splitting into a few component beams, in order to keep the technical outlay within bounds. The accuracy of rotating analyzers cannot be achieved with such polarimeters. U.S. Pat. No. 4,158,506 and EP-A-0 632 256 describe arrangements in which a plurality of detectors are placed inside the main beam. The power density, and thus the measuring sensitivity decreases strongly in a corresponding way owing to the expansion.
DE-A-19 537 706 discloses for the purpose of online monitoring of the birefringence of foils, films and sheets a rotation-free arrangement and an analyzing method in which the main beam to be analyzed is split with the aid of a two-dimensional diffracting structure into a number of component beams of the same beam profile and the same intensity, the component beams traversing elements sensitive to linear polarization and impinging on the corresponding number of detectors whose signals are subjected to Fourier analysis with respect to directions.
It is the object of the invention to create a method for continuously controlling the shrinkage of a film during taking-off from a calender, in which the shrinkage is continuously measured and account is taken of the mutual influence and/or correlation of longitudinal and transverse shrinkage and, moreover, the shrinkage is controlled within prescribed shrinkage limits. Within the scope of this object, the aim is also to measure the shrinkage of the films continuously with the aid of the dichroism and/or the birefringence over the width of the film web.